As a result of the increasingly important environmental disadvantages of the internal combustion engine, pressure continues to mount on automotive manufacturers to reduce carbon dioxide (CO2) emissions of engines of vehicles they make. To this end, vehicle manufacturers and others are developing Hybrid Vehicle (HV) technology, Electric Vehicle (EV) technology, Fuel Cell (FC) technology and Advanced Biofuel technology, amongst other technologies as a way of reducing the carbon footprint of vehicles manufactured.
In relation to HV technology, it is known for a so-called hybrid vehicle to comprise a powertrain that is controlled by a hybrid vehicle control system. The powertrain comprises an internal combustion engine and an electric motor coupled to drive wheels via a power-split device that enables the drive wheels to be powered by the combustion engine alone, the electric motor alone or both the combustion engine and the electric motor together, allowing the combustion engine to maintain a most efficient load and speed range at a given time. The electric motor is powered by a high voltage battery. A so-called “inverter assembly” is provided that comprises an inverter and a so-called “boost converter”. The inverter converts high voltage direct current from the high voltage battery of the vehicle into a three-phase alternating current for powering the electric motor. Sometimes the powertrain of the vehicle comprises more than one electric motor.
In order to provide the three-phase alternating current, the output voltage of the high voltage battery is stepped up by the boost converter from, for example, 200V to 500V. The inverter is then responsible for providing the three-phase alternating current, derived from the stepped-up voltage provided by the boost converter. In order to generate the three-phase alternative current, it is known for the inverter to comprise a bank of Insulated-Gate Bipolar Transistors (IGBTs) and parallel diodes for power modulation, the IGBTs constituting power switches.
However, for future hybrid and other electrically powered vehicles, greater demands will be made on the inverter, including low energy loss, reduced size and cost effectiveness. Furthermore, the semiconductor devices of the inverter will need to be formed from wideband gap semiconductor materials and exhibit high breakdown voltage and be able to withstand high operating temperatures.
While performance of silicon-based IGBTs is currently acceptable, these devices are less likely to perform well in respect of high current density demands, high power source voltages and high temperature operation demands that will be placed on the silicon IGBTs by future vehicle designs.
A promising candidate semiconductor material from which to fabricate power transistors is gallium nitride. However, these devices require a gallium nitride (GaN) substrate. Growth of gallium nitride substrates on a silicon substrate for subsequent separation therefrom is unfeasible due to stresses caused by lattice mismatches. In this respect, the gallium nitride layer cannot be grown sufficiently thick without the gallium nitride layer cracking when attempts are made to separate the gallium nitride layer from the silicon substrate.
To mitigate this problem, it is also known to grow the gallium nitride substrate on a Silicon Carbide substrate that has a closer lattice match with the crystalline structure of the gallium nitride grown thereon. However, the production of gallium nitride substrates of a desired thickness on Silicon Carbide substrates is costly and so a less desired manufacturing option.
As an alternative to use of Silicon Carbide, it is known to grow the gallium nitride substrate on a sapphire substrate, resulting in a more cost-effectively produced gallium nitride substrate. Indeed, a vertical power transistor device structure formed on a free-standing gallium nitride substrate grown on a sapphire substrate is described in “Vertical device operation of AlGaN/GaN HEMTs on free-standing n-GaN substrates” (Sugimoto et al, Power Conversion Conference 2007, Nagoya, 2-5 Apr. 2007). This document describes a free-standing GaN substrate having an n-GaN drift layer formed thereon. A buried-type structure having an insulated gate is then formed on the n-GaN drift layer. The gate and source are formed on a front side of the device, while a drain is formed on the back side of the device, making the device a vertical transistor device. However, such a device structure is unable to maintain a high breakdown voltage.